The present invention relates to a process controller having a plurality of modular input/output (I/O) units.
Process controllers are used to perform a variety of functions including process control functions and data-gathering functions. Process control functions include the monitoring of various conditions, such as liquid and gas pressures, flows, temperatures, etc., and responding to the state of the conditions by selectively activating pumps, valves, etc. to control the monitored conditions. The process control functions relate to such applications as oil and natural gas production and distribution, industrial plant control, etc. The data-gathering functions of process controllers allow a historical record of such variables as pressures or flows to be made over an extended period of time, such as several weeks or months, for example.
Typically, a number of process controllers are used in a computer system or network having a central host computer. For example, a single host computer may communicate with as few as 10 to more than 100 process controllers.
A conventional process controller typically contains a plurality of printed circuit (PC) boards within a housing, each of the PC boards having electronic components and circuitry to accomplish various functions. One such PC board is a controller board that controls the overall operation of the controller. The controller board typically has a microprocessor, a computer program storage memory, such as read-only memory (ROM), and a random-access memory (RAM), which are interconnected by one or more buses.
The process controller also has various types of I/O circuits, which may be provided on the controller board or on one or more dedicated I/O boards. I/O boards typically have a fixed number of various types of conventional I/O circuits. There are at least four basic types of I/O circuits: a digital input circuit, a digital output circuit, an analog input circuit, and an analog output circuit. The digital I/O circuits are used to monitor and control conditions and/or devices having only two states, on and off for example. The analog I/O circuits are used where the condition or device has many states. For example, an analog input circuit may be used to input the temperature of a liquid in a tank to the process controller, and an analog output circuit may be used to control the position of a valve having many positions.
The use of the four types of I/O circuits as described above is conventional. However, the manner in which the I/O circuits are implemented within conventional controllers has disadvantages. In particular, a specific number of I/O circuits are typically implemented on each I/O board within the controller. For example, each I/O board might have four digital input circuits, four digital output circuits, four analog input circuits, and four analog output circuits. Because the number and type of I/O circuits on each I/O board are fixed, there is no flexibility in configuring the I/O boards.
For example, suppose for a particular application a customer needed five digital input circuits and three digital output circuits. In this case, the customer would have to purchase two of the I/O boards described above in order to obtain the five digital input circuits. Moreover, the customer would not need the analog I/O circuits, but would have to purchase them anyway.
The inflexible allocation of the I/O circuits on the I/O boards also causes the control capability of the process controller to be unduly limited. A process controller typically has a maximum number of internal slots into which I/O boards can be inserted. If the controller had three I/O slots for the specific I/O board described above, that controller could only control a maximum of 12 digital input circuits, 12 digital output circuits, 12 analog input circuits, and 12 analog output circuits. If the application required 13 circuits of any particular type, an additional process controller would be required. For example, if the application required 13 digital input circuits and no other type of I/O circuits, two controllers would be required. In addition to having to purchase a second controller, the customer would have to pay for 16 of each of the four types of I/O circuits, for a total of 64 separate I/O circuits, even though only 13 I/O circuits were needed.
Even if the disadvantages described above were somehow overcome, by not including circuit components for the unwanted I/O circuits on the I/O boards for example, the conventional manner of implementing I/O circuits would suffer other disadvantages due to its inherent inflexibility.